This application claims the priority of Korean Patent Application No. 2003-29617, filed on May 10, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a local heatsink welding device and a method of welding using the same, and more particularly, to a local heatsink welding device and a method of welding using the same for improving mechanical properties of a welded part of a material.
2. Description of the Related Art
A pressure vessel for a nuclear power plant, which should be used for more than 40 years under high pressure, at high temperature, and in an atmosphere of neutron radiation, must have a high resistance to neutron irradiation embrittlement, high fracture toughness and fatigue strength, high degree of homogeneity, low susceptibility to induced radiation, low corrodibility, and good weldability, etc. In particular, in a core region of the pressure vessel, radiation of high-energy neutrons during operation causes reduction of a maximum absorption energy, and neutron radiation embrittlement increases a ductile-brittle transition temperature, thereby limiting operating conditions and a lifespan of the pressure vessel. Accordingly, to mitigate the operating conditions and extend the lifetime, it is preferable that the pressure vessel is made of a material having a high impact resistance (high fracture toughness).
Therefore, the pressure vessel for a nuclear power plant is manufactured based on a chemical composition specified in the ASME SA508 Gr. 3. SA508 Gr. 3 steel is also widely used for steam generators, pressurizers, and reactor coolant piping of nuclear power plants.
However, even if the pressure vessel is made of low alloy steel such as SA508 Gr.3 steel having the chemical composition specified in the ASME SA508 Gr.3, some of the above-mentioned requirements on the characteristics of the pressure vessel cannot be met. When manufacturing a pressure vessel with the low alloy steel, a welding process is essential. In the welding process, a local weak point at which mechanical strength and fracture toughness are degraded is generated by formation of carbide coarsening in a non-transformation heat-affected zone due to repeated heat input during multi-layer welding and postweld heat treatment. When manufacturing the pressure vessel of a nuclear power plant made of the SA508 Gr. 3 steel by a conventional method and apparatus for welding, some requirements on the mechanical/fracture properties of the pressure vessel cannot be met due to the carbide coarsening in the non-transformation heat-affected zone.
To solve this problem, a method of manufacturing a tougher SA508 Gr. 3 steel that has a remarkably improved fracture toughness is disclosed in Korean Patent Application No. 261664. The method includes performing an additional heat treatment in a temperature region at which a ferrite phase and an austenite phase coexist, between conventional heat treatment processes, i.e., quenching and tempering processes. The method enables manufacture of SA508 Gr. 3 low alloy steel having a remarkably increased room temperature impact resistance and maximum absorption energy and remarkably improved fracture toughness due to a lower ductile-brittle transition temperature, when compared to the SA508 Gr. 3 low alloy steel manufactured by conventional heat treatment. By manufacturing the pressure vessel for a power plant with the improved SA508 Gr. 3, the degradation of mechanical properties such as mechanical strength and fracture toughness resulting from a local brittle zone created during welding can be indirectly solved.
However, manufacturing a pressure vessel for a nuclear power plant using the improved SA508 Gr. 3 low alloy steel costs more and takes a longer time because of the added new heat treatment process required to make the improved low alloy steel, which is not conducive to progress in the power plant industry.